The present invention is directed to a method of impregnating and purifying polymers and copolymers with carbon dioxide or other pneumatogens.
The preparation of molded foamed plastic articles is well known wherein plastic particles are impregnated with organic blowing agents, expanded to preliminary low density particles, placed into a mold and finally expanded to the desired fully expanded article. The most used organic blowing agent is n-pentane. Unfortunately, n-pentane is now considered to be environmentally undesirable in certain geographical areas in the quantities which are removed during expansion and molding. Also, although chlorofluorocarbons are satisfactory blowing agents, these materials are undesirable because they are believed to be harmful to the earth's upper ozone layer.
To eliminate this problem, various inert blowing agents such as carbon dioxide, nitrogen, air and other pneumatogens have been tried.
U.S. Pat. No. 2,023,204 teaches to impregnate and foam polystyrene using carbon dioxide, nitrogen, sulfur hexafluoride, and the like.
U.S. Pat. No. 2,531,665 teaches to impregnate polymers at 100.degree.-125.degree. C. with inert gas under pressure followed by cooling and reheating in a mold to bring about expansion.
U.S. Pat. No. 2,848,427 teaches to impregnate polystyrene crosslinked with 0.01-0.25% DVB with CO.sub.2 under pressure at a temperature below the Tg until at least 2% CO.sub.2 was absorbed.
U.S. Pat. No. 3,243,485 teaches to impregnate films or sheets of polystyrene in dry ice.
U.S. Pat. No. 4,695,621 teaches to purify polymers with supercritical carbon dioxide.
U.S. Pat. No. 4,703,105 teaches to extract residual acrylonitrile and other low molecular weight compounds from ABS and styrene-acrylonitrile copolymers.
U.S. Pat. No. 4,911,869 teaches to mold low density foam articles by re-impregnating the expanded particles with carbon dioxide or any other pneumatogen.
U.S. Pat. No. 4,925,606 teaches to further expand extruded foam sheet by impregnating the expanded sheet with carbon dioxide.
DE 3,323,940 teaches a method of purifying poly (methacrylamide) by extracting the polymer with supercritical carbon dioxide or sulfur hexafluoride.
European Patent Application 0,183,314 teaches to purify acrylonitrile-containing copolymers by extraction of acrylonitrile, styrene, alpha-methylstyrene, and other impurities with carbon dioxide or ethylene at critical or supercritical temperature and pressure.
European Patent Application 0,281,464 teaches to purify styrene-vinylpyridine copolymers by extraction with supercritical carbon dioxide.
No one of the cited references teaches to purify, extract and impregnate the polymers in one step. Also, no one has previously taught how to combine this step with expansion and molding.